Simple because they are not good enough.
In addition, few Americans look at Formula 1. After the blunders they have beaten in the US GP’s (6 cars on the track with the Tiregate and team orders in another race)
The F1 is also less focused on spectacle and that’s what attracts most of Americans.(Spectacular crashes like Nascar and IndyCar and exciting until the last round).
Therefore, it will be difficult for a talented driver to find sponsors to get into F1.
In view of the already existing offer, they will choose the IndyCar earlier.
Drivers from the United States seldom opt for an adventure abroad.This is mainly due to two main reasons:
1: Chance to grow to F1 is extremely small.
The juniorklasses are already full of Europeans who also want a chance on their flow to the highest class.Often these drivers are supported in their growth from a young start (karting etc). An American who then enters through a detour are seen as a maverick and must already be of an extremely large caliber to stand out. And even that is not a guarantee. In addition, obtaining sponsors for financing a career in European motor racing is not easy for an American driver. The American market mainly focuses on its own market and the European couples prefer to join a Europeans.
2: The American Autosport ladder is completely different set up and provides more support
The design of the American motor sport is particularly aimed at pushing the young talents.Here in the European racing classes one often sees that drivers linger for a number of years in a junior class, because they do not get the net or are not given the right chance. In IndyCar There is the ‘ Road To Indy’ program that helps drivers with the help of scholarships and other commitments to set a step higher. For example, the winner of the class under the IndyCar (IndyCar Lights) will receive a $1 million scholarship and a guaranteed seat for three races including the Indy 500 in the year on it. Add that IndyCar, through a shootout format drivers from all sorts of racing classes (including European), gives the opportunity to join this program and it is quite logical that American drivers will rather choose a well-groomed road to the Upper class than an uncertain road in Europe. In addition, there is also the commercial side that is different than in Europe. As indicated at the first point, the American commercial market mainly focuses on its own motor sport. This delivers for example as advantage on sponsors per car in a same team can differ. This is a practice that is barely found in Europe in the single seaters. So it is also not attractive for sponsors to go there. And former drivers still remain a welcome guest with sponsors, television etc.
In general, these factors make it much more attractive for American drivers to choose a road in their own country than an adventure overseas.And the drivers who make the switch still often end up in the IndyCar. Alexander Rossi was the last American F1 driver and rode back around, he went with the idea to return the year to a year IndyCar driving and promptly won the Indy 500. He did not return and is not only a contender for the title, but also a crowd favourite. That was not so easy in F1.